Modular cross connect system with 3D-MEMS for optical telecommunication networks

ABSTRACT

A modular cross connect system for optical telecommunication networks has the optical unit divided in at least two main bodies with one section for connection comprising the collimators and a main commutation section with MEMS devices. The first section is a fixed part while the second section is a readily removable section. The two sections face each other through a window and, in the first section, optics are provided for steering all or part of the optical signals from and to the main MEMS unit to a MEMS standby or protection plane to allow replacement of the main MEMS unit without interrupting service.

BACKGROUND OF THE INVENTION

The present invention relates to an innovative telecommunication crossconnect structure based on 3D-MEMS.

Optical switching devices termed Cross Connect Switch (OXCs) for routingof optical signals in telecommunication systems are well known. The moreconventional OXCs perform routing of optical signals by first convertingthem into electrical signals which are routed to the appropriate outputport by means of electronic circuits and are then reconverted intooptical signals. The switching takes place in this manner on electricalsignals. With the progressive increase in transmission speeds, bandwidths and network complexity, conversion of signals from optical toelectrical and again optical becomes ever more difficult, costly andcumbersome.

OXCs have therefore been proposed in which switching takes placedirectly on the optical signals employing the so-called MEMS (MicroElectro Mechanical System) members. These are virtually arrays of micromirrors realized advantageously on silicon chips by techniques similarto those of integrated circuit production and which are controlledelectrically to be oriented so as to direct the optical signals towardsthe appropriate output ports. It is thus possible to perform switchingbetween a high number of ports without going through conversion of theoptical signal into electrical signals. In using MEMS systems the3D-MEMS technology appears to be the most promising.

In the more modern telecommunication networks very high data flows haveto be managed and this requires highly reliable equipment. For thisreason protection of the system is one of the main points which have tobe faced. Another important feature is low loss of insertion in order toallow completely transparent optical switching in environments with longor very long sections. The two requirements are usually in conflict withone another. Indeed, to supply adequate protection, additional opticalcircuitries with associated losses and interconnections are necessary.In addition, the additional optical circuitry adds costs to the OXCarchitecture with MEMS.

A typical architecture comprises optical switches located on the opticalcards with additional circuitry necessary for detecting optical inputand output power. The main blocks of this architecture are two MEMSunits with associated control electronics and various optical cards withthe input-output ports and associated switches and control outlets.

A heavy optical interconnection with associated high losses andcomplexity is required between all the blocks.

Management of the optical fibers is then another key point in thedevelopment of OXC.

In the prior art the functions of the optical cards are partiallyrealized even within the MEMS units to detect optical power feedback toperform fine setting of the mirrors during operation. As an alternative,more optical circuitry can be added.

However that may be, the result is always additional costs and signallosses.

The MEMS units and optical cards are often located in different racksbecause of the considerable space occupied by all the blocks. Thiscauses considerable problems in management of the optical fiberarrangement.

The general purpose of the present invention is to remedy the abovementioned shortcomings by making available an innovative OCX structurewith MEMS switching units which among other things should be modular,relatively low in cost, and easy to update and maintain while supplyingvery high performance.

SUMMARY OF THE INVENTION

In view of this purpose it was sought to provide in accordance with thepresent invention a cross connect system for optical telecommunicationnetworks comprising input and output ports which are variouslyinterconnected by means of switching members characterized in that ithas a connecting section comprising in turn said ports and a mainswitching section comprising the switching members in the form of MEMSdevices with the switching section contained in a quickly removable boxand the connecting section comprising optical switch means whichintercept on command optical paths between the two sections to deflectthem from the main switching section towards a protective switchingsection.

BRIEF DESCRIPTION OF THE DRAWINGS

To clarify the explanation of the innovative principles of the presentinvention and its advantages compared with the prior art there isdescribed below with the aid of the annexed drawings a possibleembodiment thereof by way of non-limiting example applying saidprinciples. In the drawings:

FIG. 1 shows diagrammatically an OXC with MEMS in accordance with thepresent invention,

FIG. 2 shows diagrammatically the device of FIG. 1 in case of normaloperation,

FIG. 3 shows diagrammatically the device of FIG. 1 in case of emergencyoperation after a failure of the main MEMS unit,

FIG. 4 shows diagrammatically a first solution for monitoring theoptical power at input to and output from the device,

FIG. 5 shows diagrammatically a second solution for monitoring theoptical power input to and output from the device,

FIG. 6 shows diagrammatically an assembly detail of the device inaccordance with the present invention,

FIG. 7 shows diagrammatically a second assembly detail of the device inaccordance with the present invention,

FIG. 8 shows diagrammatically a third assembly detail of the device inaccordance with the present invention,

FIG. 9 shows diagrammatically the apparatus in accordance with thepresent invention equipped with a positioning error detection system,

FIG. 10 shows diagrammatically an assembly detail of the positioningsystem of FIG. 9,

FIG. 11 shows diagrammatically an embodiment of a 1:1 OXC in accordancewith the present invention,

FIG. 12 shows diagrammatically an embodiment of an N:1 OXC in accordancewith the present invention,

FIG. 13 shows diagrammatically a variant embodiment of a part of thedevice in accordance with the present invention,

FIG. 14 shows diagrammatically a detail of the variant of FIG. 13,

FIGS. 15 and 16 show diagrammatically front and side views of a subrackcontaining a modular embodiment of an OXC applying the principles of thepresent invention, and

FIGS. 17 and 18 show diagrammatically front and side views of a rackcontaining a modular embodiment of an OXC applying the principles of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the figures, FIG. 1 shows diagrammatically by way ofexample a first embodiment of an OXC designated as a whole by referencenumber 10 in accordance with the present invention. The optical unit isdivided in two main bodies, to wit one section 11 for connection andcomprising the collimators 28 and a section 12 for switching with theMEMS. The first section is a fixed part not designed to be replaced,relatively simple and therefore reliable. The second section is areplaceable section containing the movable mirrors with MEMS technologyand associated control parts. The two sections face each other through awindow 13. As shown below, the replaceable MEMS section also comprisesdevices for accurate realignment of the structure with the collimatorarrays in the fixed part. For the sake of simplicity only one MEMSoptical path is shown diagrammatically.

The window 13 can be sealed with a glass plate with appropriateantireflection coatings or can comprise protective shutter means notshown which close upon separation of the two sections and open uponcoupling of the two sections. This second solution further reduceslosses along the path of the beams. Protective caps or films to beremoved before assembly can also be provided as alternatives.

The collimators and the MEMS units are both built and assembled withprecision and tested separately at the factory. The alignment of theMEMS with the collimators is done in the field by means of mechanicaldevices located in the MEMS unit.

In section 11 of the collimators there are optical means 14 for steeringof all or part of the optical signals from and to the MEMS section 12 toa MEMS standby or protection plane not shown in FIG. 1 for the sake ofsimplicity but similar to the unit 12.

The optical means 14 can be virtually any known type such as flat LCD,MEMS, mechanical shutters et cetera. As will be obvious to those skilledin the art, if LCD technology is employed the steering can be realizedselectively on the basis of the individual channel and increasing theversatility of the equipment.

The interface 16 with the client requires connectors for the individualfibers to be able to disconnect each beam individually. To reduce lossesthese connectors are advantageously connected to the arrays ofcollimators 15 without any further internal connector. This lossreduction can compensate for the introduction of the steering means 14.

FIG. 2 shows the device 10 during normal operation. When the flat MEMSoperates normally the steering means 14 offer full transparency to thebeams between the MEMS unit 12 and the collimators 15.

It is advantageous that only the optical part of the steering devices belocated in the collimator block while no electronic part is located insaid block.

As clarified below, input and output optical power can be detected withan outlet on each input and output fiber although this involves anincrease in design complexity or using a CCD device under the fixedcentral mirror in the MEMS structure, realizing said mirrors withpartial reflection or as an alternative additional semitransparentmirrors can be provided in the collimator section or the MEMS. In anycase it is advantageous that the electronics be kept out of thenonremovable sections to improve reliability.

In case of problems in the MEMS section, which can be detected by theabove mentioned power monitoring devices, the steering means 14 areactivated to offer full reflection to the beams from and to thecollimators so as to insulate the main MEMS section from the opticalpaths. This is shown in FIG. 3.

Under this condition the main MEMS section can be removed and replacedby a new unit while the switching functions are assured by theprotection plane towards which the means 14 deflect the optical paths.

FIG. 4 shows diagrammatically a first solution for detection of the MEMSunit input and output signal power. In this solution partiallyreflecting mirrors 17 an CCD devices 18 are provided in the MEMSsection. This way, a small part of the beams from and to the MEMSnucleus are sent to the CCDs which allow the electronics to make inputand output power measurements and detect any anomalies or failures.

FIG. 5 shows a variant in which a similar concept is applied in thecollimator section 11. In this second case the CCD devices 18 areilluminated by semireflecting mirrors 17 which can coincide with thesteering means 14 appropriately realized and controlled. Advantageouslythe CCD sensors can be realized to be easily replaceable for improvingsystem reliability.

As mentioned above, finely accurate positioning is required foralignment of the MEMS section with the collimator sections. Positioningshould be accurate in approach direction (Z) of the two sections and inthe two directions (X,Y) transversal to said approach direction.

In direction Z there may be provided a simple system of running onguides for approach of the two sections with a mechanical stop at theend of travel to lock the two sections together with the desiredprecision, for example with an accuracy near 100 μm. FIG. 6 showsdiagrammatically a similar running structure in which the container 12of the MEMS section runs along guides 20 extending in the approachdirection Z and projecting from the section or unit 11 of thecollimators.

FIGS. 7 and 8 show a diagram of fine positioning of the MEMS unit alongaxis X. In this embodiment the planes of the mirrors and the associatedaccessorial optical parts are fixed on a single plate 21 with lowthermal drift. As may be seen in the figures, the plate can move inrelation to the MEMS unit container along directions X and Y by means ofactuators 22, 23 of suitable precision and controlled by an appropriatecontrol logic 24 in the container.

The precision traversing plates can be roughly positioned manually firstand then finely by means of an end adjustment section, for examplepiezoelectric, for in-line position control.

For control of positioning errors error detection systems could beconsidered for error detection with feedback control of plate position.

The sources of errors may be diverse. But it may be assumed that theangular errors of alignment and the errors along the axis z are limitedby the mechanical precision.

In addition, the angular errors are intrinsically compensated by theoscillating structure of the MEMS mirrors while errors along axis z arenot critical if compared with the travel of the optics.

The critical errors which need compensation through a control system aretherefore essentially those along axes x and y.

Naturally additional position controls can be added to compensate forunexpected errors.

FIG. 9 shows a possible advantageous system of position error detection.This system comprises a laser source 24 fastened to the subunit of thecollimators and producing luminous beams pointed at the fixed mirrorslocated on the plane of the MEMS mirrors. The beams are then reflectedonto a sensor 25 with four quadrants as seen better in FIG. 10 in theX-Y plane and located on the MEMS unit support plate. The basicassumption is that the collimator and MEMS modules are each assembledaccurately and only the relative positioning is unknown.

The control logic 24 controls the x-y positioning actuators by means ofa closed loop to compensate continuously for mechanical shifts due forexample to aging of the components and temperature drifting.

FIG. 11 shows diagrammatically the simplified embodiment of a 1:1 OXC inwhich the MEMS work plane 12 and the MEMS protection plane 12 b are bothconnected to the same collimator unit. For the sake of simplicity onlyone beam direction is shown. In the embodiment of FIG. 11 another returnmirror 30 is added to enable mounting of both MEMS modules on the sameside of the basic collimator module. As may be seen, this arrangementallows realization of a complete and compact OXC with two removableintegrated MEMS matrices. Insertion losses are very much reduced toavoid additional connectors and/or collimators. The interface with theclient can be realized by means of multifiber tape connectors. Access tosingle channels can be achieved by means of single fiber connectorslocated on dedicated frames.

A rack structure of the assembly can be realized for example with a rackcontaining at the rear the collimator unit and at the front the two MEMSmodules so that the latter can be readily drawn out and replaced in caseof need.

FIG. 12 shows diagrammatically a modular OXC similar to the one of FIG.11 but with generalization to N modules. For the sake of simplicity, thefigure is limited to N=4 and only the input direction of the beams inthe MEMS is shown. As may be seen in the figure, a single 3D-MEMS modulecan be used to protect any one of the remaining MEMS planes. Thecollimators can be sealed in a single unit supplied with horizontalwindows optically connecting the various MEMS units, each in its ownremovable container, to the collimators.

In the various embodiments and in particular in the N:1 OXC embodimentit may be advantageous to place the various steering means 14 within thecollimator assemblage and if necessary seal all in dedicated containerseasy to replace. FIG. 13 shows a similar embodiment with a modularcontainer 31 containing the collimator array 28 and the steering device14 and which is equipped with the front window 13 for optical connectionwith the MEMS module and opposing side windows 32 to allow the deflectedbeams to traverse the containers.

The containers 31 can be packed together side-by-side to realize thestructure of FIG. 12.

As may be seen again in FIG. 13, each unit 31 can be equipped with alaser 33 to supply an alignment beam for the side-by-side modules. Asshown in FIG. 14, which shows the side view of the module shown in planview in FIG. 13, the collimator, laser and steering device can befastened onto the same substrate which provides a frame movable on thehorizontal plane by means of precision traversing plates with ifnecessary electrically adjustable operating means 35, 36 to allowaccurate alignment of the modules using the alignment laser 33 and ifnecessary an appropriate receiving sensor (not shown). The precision ofthe vertical direction position (Y) is ensured by the precision of themechanical framework which receives the modules.

FIGS. 15 and 16 show an advantageous arrangement of the modular membersinside a subrack. As seen, there is the collimator unit (notreplaceable) and drawer units for each steering device 14 and each MEMS12, 12 b.

Naturally if being able to quickly replace the steering devices is notconsidered useful these devices can be inserted permanently in thecollimator unit 11.

If the steering devices are in replaceable modules, appropriate windows40 are provided for optical connection to the other modules.

FIGS. 17 and 18 show diagrammatically the possible advantageousarrangement of parts in a rack to realize a complete OXC system inaccordance with the present invention. For the sake of simplicity a 1:1OXC embodiment is shown. On the basis thereof the N:1 embodiment can bereadily imagined by those skilled in the art.

As may be seen in the figures, from top to bottom there are providedsuperimposed main control and operating cards 40, control and operationstandby cards 41, connectors 42 for the client interface (LC or MU), themain removable MEM modules 43 and the removable protective MEM modules44.

It is now clear that the predetermined purposes have been achieved bymaking available modular OXCs with MEMS and with high performance, easymaintenance and expansion and relatively low costs.

Naturally the above description of an embodiment applying the innovativeprinciples of the present invention is given by way of non-limitingexample of said principles within the scope of the exclusive rightclaimed here.

For example the steering means arranged in the collimator units can berealized removable, for example supported in a third section separablefrom the other two to facilitate repairs. The connector units in thecollimator section can be realized with known coupling systems forselective withdrawal and replacement thereof without mutualinterference.

1. A cross connect system for optical telecommunication networks,comprising: input and output ports interconnected by means of switchingmembers; and a connecting section including, in turn, said ports and amain switching section having the switching members as MEMS devices, theswitching section being contained in a removable box, and the connectingsection including optical steering means for intercepting on commandoptical paths between the two sections from and to the connectingsection to steer them from the main switching section towards aprotective switching section, the MEMS devices being supported in thebox by a support system arranged to provide controlled movement of theMEMS devices for alignment of the MEMS devices with the optical paths.2. The system in accordance with claim 1, in that the protectiveswitching section comprises additional switching members as MEMS deviceswhich are housed in an additional removable box.
 3. The system inaccordance with claim 2, in that at least the switching means are housedin a removable box for replacement.
 4. The system in accordance withclaim 1, and sensors in the switching section, for detecting inputoptical power to, and output optical power from, the main switchingsection.
 5. The system in accordance with claim 4, and sensors in theconnecting section, for detecting input optical power to, and outputoptical power from, the main switching section.
 6. The system inaccordance with claim 5, in that the sensors are illuminated CCDsintercepting part of the optical paths with semireflecting mirrors. 7.The system in accordance with claim 1, in that the switching sectionsare a plurality, and in that the connecting section comprises a unitmade up of an array of collimators and optical steering means for eachswitching section.
 8. The system in accordance with claim 7, in thateach collimator array and optical steering means unit is in turn housedin a replaceable box.
 9. The system in accordance with claim 8, in thateach removable box comprises means for alignment detection and means foralignment adjustment.
 10. The system in accordance with claim 7, in thatat least the steering means of each unit are housed in replaceableremovable boxes.
 11. A telecommunication system comprising a crossconnect system in accordance with claim
 1. 12. The system in accordancewith claim 1, in that the support system is arranged to align theoptical paths in an approach direction and in two directions transverseto the approach direction.
 13. The system in accordance with claim 1,further including guides extending in an approach direction to theconnecting section along which the switching section is arranged to runto provide said alignment
 14. The system in accordance with claim 1, inthat the MEMS devices include planes of mirrors, and in that the systemincludes a single plate on which the planes of mirrors are fixed. 15.The system in accordance with claim 14, in that the plate is arranged tobe moveable in relation to the switching section in two directionstransverse to an approach direction.
 16. The system in accordance withclaim 15, including actuators for moving the plate.
 17. The system inaccordance with claim 16, including control logic for controlling theactuators.
 18. The system in accordance with claim 17, in that thecontrol logic is arranged to control actuators by means of a closed loopto compensate continuously for mechanical shifts.
 19. The system inaccordance with claim 1, further comprising a laser position sensingsystem for sending a laser beam between the connecting section and theswitching section for verification of the alignment.
 20. The system inaccordance with claim 19, in that the laser position sensing systemincludes a position sensor which comprises four quadrants for monitoringa position of the laser beam.
 21. A cross connect system for opticaltelecommunication networks, comprising: input and output portsinterconnected by means of switching members; and a connecting sectionincluding, in turn, said ports and a main switching section having theswitching members as MEMS devices, the switching section being containedin a removable box, and the connecting section including opticalsteering means for intercepting on command optical paths between the twosections to steer them from the main switching section towards aprotective switching section, the optical steering means being arrangedon optical paths between collimators and optical connection windows withthe main switching section.
 22. A cross connect system for opticaltelecommunication networks, comprising: input and output portsinterconnected by means of switching members; and a connecting sectionincluding, in turn, said ports and a main switching section having theswitching members as MEMS devices, the switching section being containedin a removable box, and the connecting section including opticalsteering means for intercepting on command optical paths between the twosections to steer them from the main switching section towards aprotective switching section, the main switching section box and theprotective switching section box being arranged side by side and facingeach other with a respective optical path passage window on acorresponding connecting section window.